Question 981916
Rate of filling with just the hose:
[ 1 pool filled ] / [ 120 min ]
Let {{{ r }}} = the rate that the 
drain by itself empties the pool
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Add the rates of the hose and the drain
open together, but make the drain's rate 
negative ( emptying )
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{{{ 1/120 - r = 1/360 }}}
{{{ r = - 3/360 + 1/360 }}}
{{{ r = -1/180 }}} ( it's negative, as it should be )
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So, the drain by itself empties 1 pool in
180 min by itself
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Check:
{{{ 1/120 - r = 1/360 }}}
{{{ 1/120 - 1/180 = 1/360 }}}
{{{ 3/360 - 2/360 = 1/360 }}}
{{{ 1/360 = 1/360 }}}
OK